Sedimentology of Shallow, Hurricane-Affected Lagoons: Grand Cayman, British West Indies

William B. C. Kalbfleisch, Brian Jones

Abstract


Frank Sound and Pease Bay are small narrow (~4 km long and <1 km wide) shallow water (1.5-2.0 m average depth) lagoons (< 0.5 m deep) located on the exposed-windward margin of south coast of Grand Cayman. Collectively, the Rubble and Knob, Bare Sand, and Thalassia and Sand zones form 95-97% of the substrates in these lagoons. The Bare Rock Zone, Coral Knolls, and Shoreline Zone are restricted to small areas in both lagoons. Between 1985 and 1992, the area covered by Thalassia expanded by colonizing the Bare Sand Zone.

During fair-weather conditions, onshore wave energy is dampened by the reef. Nevertheless, waves that cross the reef are sufficient to maintain lagoonal circulation and "normal" marine conditions. Under these conditions, there is limited sediment production (e.g., Thalassia epibionts, bioerosion, green algae), active bioturbation, and expansion of the Thalassia banks. Sediment transportation is minimal. At the height of a hurricane, waves and currents pass over the reef and produce a turbulent sediment laden currents that cross the lagoons. As the current loses energy, deposition produces a sediment wedge that grades from boulders-cobbles near reef crest, to pebbles and coarse sand ~80 m from the reef crest, to fine sand 500-600 m from the reef crest. The sediment is poorly sorted and typically has a unimodal grain size distribution. Silt and clay sized sediment is rare. Skeletal constituents indicate that most sediment originates in the fore-reef and shelf environs. As the storm wanes, water that was piled in the lagoons during the storm starts to drain out through topographically controlled mega-rip currents. These high velocity currents destroy biota, uproot Thalassia, and strip sediment to reveal bare rock substrates.


Keywords


Coral reef; bioturbation; seagrass; coastal environments; grain-size analysis; bottom types

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